The invention relates to a block-resistant film. More precisely, the invention relates to a multilayer film having a core layer and a block-resistant layer which inhibits blocking to a functional layer of the film which is printable or sealable or treatable for sealing or printing.
Blocking is the unwanted adhesion between layers of plastic film that may occur under pressure, usually during storage or use. It is known that blocking can be prevented with the use of antiblocking agents which are added to the composition which makes-up the surface layer of the film. Known antiblocking additives for plastic packaging film include synthetic waxes. In U.S. Pat. No. 4,692,379, various antiblocking agents for a heat sealable outer skin of a multilayer film are specifically described, including silica, clay, talc and glass.
Sealable coatings are used on flexible packaging films so that the films can be sealed with the application of pressure, with or without out exposure to elevated temperatures. These so called xe2x80x9ccold sealxe2x80x9d coatings can pose blocking problems. A typical cold seal coating is a natural or synthetic rubber latex combined with a soft polymer which tends to be tacky at room temperature and causes blocking. The rubber component permits sealing with slight pressure and without using heat. The cold seal coating is usually applied to a plastic film as it is wound into a roll. Since the cold seal coatings are tacky, it is important that the backside of the film which contacts the cold seal coating upon winding does not stick (block) to the cold seal coating so that the film can be easily unwound for use on packaging equipment.
One approach for reduced blocking between the cold seal coating and the backside of the film has been to formulate a cold seal coating which is nonblocking to certain surfaces including polypropylene, such a cold seal formulation is described in U.S. Pat. No. 5,616,400.
Another approach uses a cold seal release material on the layer opposite the cold seal surface. See U.S. Pat. Nos. 5,482,780; 5,489,473 and 5,466,734.
In U.S. Pat. No. 4,692,379, a film is described which has an upper heat sealable layer formed from an ethylene-propylene-containing copolymer or terpolymer and an antiblocking agent, the lower heat sealable layer is formed from an ethylene-propylene-containing copolymer or terpolymer and antiblocking agent and a quantity of silicone oil such that the coefficient of friction-reducing amount of the silicone oil will be present on an exposed surface of the upper heat sealable layer following mutual contact of the upper and lower surfaces. The silicone oil additive is described as having a viscosity of from about 350 to about 100,000 centistokes with about 10,000 to about 30,000 centistokes being preferred. An advantage of the invention as described in the ""379 patent is that the silicone is present on the exposed surface of the lower layer in discrete microglobules which, to some extent, transfer to the upper surface upon contact. The silicone on the surfaces of the film facilitates machinability.
An attempt was made to produce a block-resistant functional film, typically a film having a printing function or sealing function, with silicone oil in a surface layer as an antiblocking agent. It was found that the silicone oil was detrimental to the printing or sealing function.
Copending U.S. application Ser. No. 09/026,454, U.S. Pat. No. 6,074,762 filed Feb. 19, 1998, describes a multilayer film having a core layer and a block-resistant layer which inhibits blocking to a functional layer of the film which is printable or sealable or treatable for sealing or printing. The block-resistant layer includes polydialkylsiloxane, preferably in small quantities. When the film is wound into a roll, polydialkylsiloxane deposits silicone onto the functional layer but the amount of silicone deposited is not substantially detrimental to the printing function or the sealing function.
The invention is directed to a block-resistant film, comprising:
(a) a core layer of a thermoplastic polymer, the core layer having a first side and a second side,
(b) a functional layer which is printable or sealable or treatable for printing or sealing on the first side of the core layer, and
(c) a block-resistant layer on the second side of the core layer comprising a thermoplastic polymer and an amount of a polydialkylsiloxane, based upon the entire weight of the block-resistant layer, sufficient to inhibit blocking of the block-resistant layer to the functional layer when they are in contact, wherein the surface of said block-resistant layer is flame treated or corona treated.
It is a feature of the invention to have a block-resistant layer which comprises a thermoplastic polymer and an amount of a polydialkylsiloxane sufficient to inhibit blocking of the block-resistant layer to the functional layer which happens when the film is wound into a roll or stacked so that the functional layer is in contact with the block-resistant layer.
It is an advantage of the invention that when the film is wound into a roll or stacked for storage in such a manner that the functional layer is in contact with the block-resistant layer of the film, the block-resistant layer, made with the polydialkylsiloxane described herein, deposits less silicone onto the functional layer than the silicone oil described in U.S. Pat. No. 4,692,379 such that with the instant invention the printing or sealing function is not substantially impaired.
The block-resistant film of the instant invention is resistant to the undesirable adhesion between an exposed surface of the first side of the film to an exposed surface of a second side of the film which adhesion develops under pressure, typically, during storage or use.
The block-resistant layer of the instant invention is made with a particular polydialkylsiloxane additive. The polydialkylsiloxane additive is especially selected because it inhibits blocking and does not tend, in an amount detrimental to the functional properties of the film, to appear on the surface of the block-resistant layer or the functional layer. The appearance of the polydialkysiloxane may be determined by measuring the amount of silicon on the film surface by Electron Spectroscopy for Chemical Analysis (ESCA). It has been found that when a functional layer which is printable or sealable or treated for printing or sealing, contacts a layer which is compounded with a polydialkylsiloxane, of the kind described in U.S. Pat. No. 4,692,379, that polydialkylsiloxane (silicone oil) appears on the surface of both the functional layer and the block-resistant layer, after contact of the functional layer to the block-resistant layer (by measuring the amount of silicone), to an extent which is considered detrimental to the printability or sealability of the functional layer.
The core layer comprises a film-forming thermoplastic polymer which has properties suitable for extrusion or coextrusion followed by biaxial orientation in the machine and transverse directions under elevated temperature so as to form a film. Although, preferably, the thermoplastic polymer of the core layer is a propylene homopolymer, it can be any polymer made from a 2 to 4 carbon atom olefin, such as ethylene or butene-1 or a polymer made predominantly of propylene with a minor amounts of another olefin, usually a 2 to 4 carbon atom olefin.
The layer which is block-resistant and the functional layer may be the same or different. The block-resistant layer and the functional layer comprise a film-forming polymer having properties suitable for extrusion and uniaxial or biaxial orientation (by stretching the extrudate in the machine direction and/or transverse direction under elevated temperatures) and for forming skin layers on the outer surfaces of the core layer. Such layers may comprise a thermoplastic polymer composed predominantly of an olefinic polymer such as polypropylene or polyethylene. A blend of polymers may also be used to form these layers.
The block-resistant layer may comprise a predominant proportion of propylene or ethylene and may contain a minor amount of another olefinic monomer having 2 to 8 carbon atoms. The block resistant layer may comprise a propylene homopolymer, ethylene homopolymer, copolymer of ethylene and propylene or a terpolymer of ethylene, propylene and butene-1.
The thermoplastic polymer of the functional layer may be the same as the thermoplastic polymer of the block-resistant layer or it may be different. Examples of particular polymers of the functional layer include copolymers or terpolymers of ethylene, propylene and butylene or another olefin having 5 to 10 carbon atoms or a mixture of these olefin polymers. When it is desirable for this layer to be printable or sealable or treatable for printing or sealing, it may comprise an ethylene homopolymer having a density of about 0.91 to about 0.96 g/cm3, ethylene-propylene copolymer in which the ethylene content is about 2 to 10% by weight based upon the total weight of the copolymer or an ethylene-propylene-butene-1 terpolymer which has about 0.5 to about 7 weight % ethylene and about 5 to about 30 weight % butylene, each based upon the total weight of the terpolymer.
The block-resistant layer is compounded with an amount of a polydialkylsiloxane sufficient to inhibit blocking of the block-resistant layer to the functional layer when the layers are in contact so that the layers can be easily separated. Such blocking characteristics are beneficial in high speed machinery, e.g., packaging machinery and printing machinery. Although analysis by electron spectroscopy may reveal that silicone from the polydialkylsiloxane is present on the surface of both the block-resistant layer and the functional layer, the amount is preferably not substantially detrimental to the printing function or sealing function of the film. For example, on winding the film into a roll for storage so that the external surface of the block-resistant layer contacts the external surface of the functional layer, the polydialkylsiloxane incorporated into the block-resistant layer has less tendency to transfer to the functional layer than the silicone oil described in U.S. Pat. No. 4,692,379. This tendency was determined by measuring the silicone present on the surfaces of the block-resistant layer and the functional layer by electron spectroscopy after the roll was unwound, as described in copending U.S. application Ser. No. 09/026,454, filed Feb. 19, 1998. U.S. Pat. No. 6,074,762.
The polydialkylsiloxane of this invention can have a number average molecular weight above about 250,000, typically above about 300,000 and a viscosity of above about 10,000,000 cSt, usually ranging from about 15,000,000 to about 20,000,000 cSt.
The alkyl group of the polydialkylsiloxane usually ranges from 1 to about 10 carbon atoms, more usually from 1 to about 3 carbon atoms, which carbon atoms can be in a straight or branched chain configuration.
One particular kind of polydialkylsiloxane used in this invention is referred to as xe2x80x9csilicone gumxe2x80x9d, also described as an xe2x80x9cultra high molecular weight siliconexe2x80x9d. Silicone gum can be in the form of a silicone polymer dispersed in polypropylene or polyethylene. Silicone gum of this kind is available in a masterbatch form from the Dow Coming Corporation, of Midland Michigan, under the product designations xe2x80x9cMB50-001xe2x80x9d and xe2x80x9cMB50-002xe2x80x9d.
The ratio of polydialkylsiloxane to polyolefin in the block-resistant layer is sufficient to inhibit blocking of the block-resistant layer to the functional layer when the layers are in contact as would happen when the film is wound into a roll. The ratio of the polydialkylsiloxane to the polyolefin will vary, for example, depending upon the nature of the polydialkylsiloxane and the nature of the olefinic component. As an example of a broad range, the polydialkylsiloxane content may range from about 1 to about 10 weight percent. In certain situations, fairly high polydialkylsiloxane contents, such as about 2 to about 10, or even about 5 to about 10, weight percent based on the total weight of the block-resistant layer, are preferred. By way of contrast, the preferred polydialkylsiloxane content in copending U.S. application Ser. No. 09/026,454, U.S. Pat. No. 6,074,762 filed Feb. 19, 1998, is between about 0.05 and about 1 weight percent based on the total weight of the block-resistant layer.
The functional layer includes those thermoplastic polymer skin layers suitable for being treated for printing or sealing or by being a thermoplastic polymer which is printable or sealable without treatment. In this regard, the functional layer may be selected from the group consisting of ethylene or propylene homopolymer, ethylene-propylene copolymer or ethylene-propylene terpolymer or a mixture of two or more of such homopolymer, copolymer or terpolymer. In one embodiment of the invention, the surface of the functional layer is treated by surface treatment which improves wettability and adhesion of printed matter (print may be by lithography, liquid or dry toner, thermal dye, dye sublimation, etc.). In another embodiment of the invention, the functional layer is a cold seal adhesion promoting layer.
It is contemplated that the block-inhibiting layer will provide excellent antiblocking properties to any commercially applied cold seal receptive composition or print receptive surface.
The functional layer may itself be printable or sealable or may be treated so as to provide sealability and/or printability. This includes surface treatment of any kind known to enhance the surface tension properties such as flame or corona treatment. Other treatment methods include the application of a printable or sealable covering layer by any means including conventional extrusion or coating. Certain water based coatings are known for their utility as cold-sealable coatings or printable coatings. Examples include acrylic-based coatings including alkyl acrylate polymers and copolymers.
Sometimes it is useful to enhance film properties or provide the film with certain properties by use of appropriate film additives. Such additives are used in effective amounts, which vary depending upon the property required, and may be selected from the group consisting of: antiblock, slip additive, antioxidant additive, moisture barrier additive or gas barrier additive.
Antistatic additives may be used in amounts ranging from about 0.05 to about 3 weight %, based upon the weight of the layer. Such antistatic additives include alkali metal sulfonates, polyether-modified polydiorganosiloxanes, polyalkylphenylsiloxanes and tertiary amines.
Antiblock additives used in amounts ranging from about 0.1 weight % to about 3 weight % based upon the entire weight of the layer. Such additives include inorganic particulates such as silicon dioxide, e.g., a particulate antiblock sold by W.R. Grace under the trademark xe2x80x9cSipernat 44xe2x80x9d, calcium carbonate, magnesium silicate, aluminum silicate, calcium phosphate, and the like, e.g., Kaopolite. Another useful particulate antiblock agent is referred to as a non-meltable crosslinked silicone resin powder sold under the trademark xe2x80x9cTospearlxe2x80x9d made by Toshiba Silicone Co., Ltd. and is described in U.S. Pat. No. 4,769,418. Another useful antiblock additive is a spherical particle made from methyl methacrylate resin having an average diameter of 1 to 15 microns, such an additive is sold under the trademark xe2x80x9cEpostarxe2x80x9d and is commercially available from Nippon Shokubai. Experimental results referred to in copending U.S. application Ser. No. 09/026,454, filed Feb. 19, 1998, U.S. Pat. No. 6,074,762 indicate that with addition of particulate antiblock additives such as Epostar MA1010, T145, Kaopolite 1152, Sipernat 44 do not materially effect antiblock properties of the block-resistant layer formulated in accordance with this invention but do reduce the coefficient of friction properties of the film.
Slip additives include higher aliphatic acid amides, higher aliphatic acid esters, waxes and metal soaps. Such slip additives may be used in amounts ranging from about 0.1 to about 2 weight percent based on the total weight of the layer. A specific example of a fatty amide slip additive is erucamide.
Antioxidants may be used in amounts ranging from about 0.1 weight % to about 2 weight percent, based on the total weight of the layer, phenolic antioxidants. One useful antioxidant is commercially available under the trademark xe2x80x9cIrganox 1010xe2x80x9d.
Barrier additives may be used in useful amounts and may include low-molecular weight resins, hydrocarbon resins, particularly petroleum resins, styrene resins, cyclopentadiene resins and terpene resins.
Optionally, the outer layers may be compounded with a wax for lubricity. Amounts of wax range from about 2 to about 15 weight % based on the total weight of the layer.
The process of making the block-resistant layer can be by masterbatch in which a minor proportion of a concentrated composition of polydialkylsiloxane and the thermoplastic polymer, along with any optional additives is prepared and mixed (for example, by dry mixing or melt compounding) into a major proportion of a thermoplastic polymer. The amount of concentrate in the masterbatch usually ranges from about 5% by weight to about 50% by weight based on the total weight of the block resistant layer. The mixture is then melt mixed in an extruder or compounded in a twin screw extruder. Alternatively, the block resistant layer is prepared in one step by mixing the thermoplastic polymer, polydialkylsiloxane and, optionally, any additives in the proportions used for making up the final skin composition.
The film may be formed by coextruding the thermoplastic polymer core layer together with the block-inhibiting layer and functional layer through a flat sheet extruder die at a temperature ranging from between about 200 to about 250xc2x0 C., casting the film onto a cooling drum and quenching the film. The sheet is then stretched about 3 to about 7 times in the machine direction (MD) orienter followed by stretching about 5 to about 10 times in the transverse direction (TD) orienter. The film is then wound onto a reel. The external surface of the block-resistant layer and, optionally, the functional layer is flame treated or corona treated before winding. A suitable coating may be applied to the functional layer.
In general, the film of the instant invention may comprise at least three layers: the core layer; the block-resistant layer (an outermost skin layer); and the layer which is functional (an outermost skin later). It is contemplated that additional layers can be incorporated between the core layer and the outermost skin layers, e.g., tie layers comprising polypropylene or polyethylene. The core layer usually represents about 70 to about 90 percent of the thickness of the total film. The skin layers are usually coextensively applied to each major surface of the core layer, typically by coextrusion, as noted above.